While the recently tightened regulation on exhaust gas has been improving in reducing discharged amounts of harmful substances such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) from an engine itself; a three-way catalyst, which is the main current at present, has also been improving. Both of them have been effective in reducing a discharged amount of harmful substances.
However, an amount of harmful substances discharged right after an engine has started is highlightened, while discharged substances are reduced extending over the whole running condition of an engine as the improvement according to tightening of exhaust gas regulations. For example, in FTP-75 cycle, which is a regulated running cycle in U.S., 60-80% of total emission discharged in the whole running cycle is discharged in the Bag-1 mode for 140 second right after the engine has started. This is because a catalyst is not sufficiently activated since temperature of exhaust gas is low right after an engine has started (Bag-1A), thereby passing harmful substances through the catalyst.
Therefore, some measures are employed, for example, putting a catalyst as close to an engine as possible in a place where exhaust gas has high temperature to raise temperature of the catalyst right after an engine has started, thinning the cell partition walls to decrease heat capacity of a catalyst itself, and increasing cell density of a carrier to quickly absorb heat of exhaust gas and to increase a contact area of a catalyst with exhaust gas.
As a catalyst, there is generally used a catalyst produced by loading γ-alumina of a fine porous structure having a high surface area on the surface of cell partition walls of a ceramic honeycomb structure, which is one of cell structures, and then noble metals such as platinum, palladium, and rhodium are loaded, as catalyst components, on the alumina. Further, to these noble metals are added ceria, zirconia, and the like, to store and release oxygen contained in exhaust gas. Such noble metals and oxygen-storing substances are present in a dispersed state in the pores in the γ-alumina layer loaded on the surface of porous cell partition walls (rib) of the carrier.
A honeycomb structure is generally used in such a condition that it is housed (canned) in a container made of metal such as stainless steel with being held by the container. In addition, a honeycomb filter obtained by alternately plugging the honeycomb structure at each end face in such a way that it looks checkerboard patterns is suitably used also as a filter for capturing and removing particulate matters contained in dust-containing fluid such as diesel engine exhaust gas (such a filter may hereinbelow be referred to as “DPF.”), and the filter is disposed in a predetermined place after being canned similarly to the case of the aforementioned honeycomb structure.
Upon canning, an appropriate compressible elastic member is disposed in a gap between the container and a peripheral surface of the honeycomb structure to impart an adequate compressing surface pressure to the honeycomb structure. An example of related prior art is a method of canning a honeycomb structure in a metal container with holding the honeycomb structure with a mat of an intumescent material containing vermiculite (see U.S. Pat. Nos. 5,207,989 and 5,385,873).
However, in the case of the method disclosed in the above U.S. Pat. Nos. 5,207,989 and 5,385,873, compressing surface pressure is rapidly raised by intumescence. Therefore, the rapidly raised compressing surface pressure tends to exceed strength (isostatic strength) of a honeycomb structure having thin walls with low strength, and the honeycomb structure is liable to break. In addition, since compressibility of an intumescent mat is quickly deteriorated from about 800 degree C., compressing surface pressure disappears at about 1000 degree C., and it becomes impossible to hold the honeycomb structure.
Whereas, in a non-intumescent mat not containing vermiculite (see U.S. Pat. Nos. 5,580,532 and 2,798,871), the change in surface pressure according to temperature-rise is very small, and the honeycomb structure can be held with surface pressure being hardly decreased even at 1000 degree C.
A honeycomb structure having thin walls has conventionally been held using a non-intumescent mat in place of an intumescent mat. However, when a honeycomb structure is wound with a mat serving as a holding member followed by being canned in a metal container, slippage tends to be caused at the joint of the mat, and surface pressure tends to be increased. Further, when a honeycomb structure having a mat wound thereon is stuffed in a metal container, the mat tends to have rumples, and surface pressure tends to be increased at that point. These cause non-uniform distribution of compressing surface pressure acting on a peripheral surface of the honeycomb structure. When partially heightened compressing surface pressure exceeds isostatic strength of the honeycomb structure, the cell structure breaks. In addition, because of the non-uniform distribution of the surface pressure, the cell structure tends to slip due to vibrations of an engine or pressure of exhaust gas in practical use.
Incidentally, “isostatic strength” of a honeycomb structure means a value measured by “isostatic fracture strength test” provided for by the automobile standards JASO standard M505-87 published by Society of Automotive Engineers of Japan, Inc. Specifically, the test is conducted in such a manner that a cell structure as a carrier is put in a rubber tube, and the container is capped and subjected to isotropic pressure compression, which imitates compression load in the case that a carrier is held at a peripheral surface thereof by a can of a converter. The isostatic strength is shown by a value of pressure at the time of breakage of a carrier. A catalyst converter for purifying automotive exhaust gas generally employs a canning structure in which a carrier is held at a peripheral surface thereof. It is a matter of course that high isostatic strength is preferable in view of canning.
When the actual surface pressure becomes higher than the intended surface pressure planned upon design of canning, the structure may break at the point if the surface pressure exceeds isostatic strength of the honeycomb structure. According as thickness of cell partition walls decreases and strength of the structure is lowered, it is necessary to decrease the intended surface pressure, and it is necessary to minimize fluctuation of the surface pressure by suppressing extraordinary increase of actual canning surface pressure. It is ideal that the actual surface pressure is equal to the intended surface pressure because it makes possible the canning design just as aimed.
Further, a honeycomb structure may break because of varied gap between the honeycomb structure and the metal container due to precision of an external shape of the honeycomb structure or because of uneven compression pressure act on the peripheral portion of the honeycomb structure and high holding surface pressure acts partially as a result of slippage of a holding member caused when the honeycomb structure is housed in a metal container. As the partition walls of a honeycomb structure are made thinner, the isostatic strength level of the honeycomb structure becomes lower, which requires to make compressing surface pressure of the honeycomb structure as low as possible with keeping the minimum surface pressure required for holding a honeycomb structure. As the level of compressing surface pressure is lowered, it is necessary to make variance in surface pressure smaller, i.e., to give more uniform distribution of surface pressure.
The present invention has been made in view of the problems of the prior art and aims to provide a honeycomb structure which is capable of being housed in a metal container under a safely held condition and which hardly has problems such as breakage or breakdown, as well as a canning structure which has a metal container housing the honeycomb structure and which is superior in vibration resistance particularly under high temperature conditions.